1. Field of the Invention
This invention relates to equipment, transponders and methods for optical fiber transmission. Specifically, it relates to a WDM (Wavelength Division Multiplexing) optical fiber transmission device and wavelength division multiplexing system. More specifically, it relates to a wavelength division multiplexing optical fiber transmission system which multiplexes plural optical signals of different wavelength in an optical fiber to transmit information, and to a transponder device or a wavelength division multiplexing optical fiber transmission device using this system.
2. Description of the Related Art
Wavelength division multiplexing (WDM) is an extremely useful technique for increasing the volume of optical fiber communications. A typical example of the construction of a prior art wavelength division multiplexing optical transmission system comprises a wavelength division multiplexing optical transmission device 152 (transmitting side) and a wavelength division multiplexing optical transmission device 153 (receiving side) disposed at sites 141-1, 141-2 located at two points separated by a distance of several kilometers to several thousand kilometers, as shown in FIG. 5. The two devices are connected by inter-site optical fiber circuits 144-1, 144-2, and a wavelength division multiplexing optical repeater 151. The figure shows only transmission from the site 141-1 to the site 141-2, although this is generally combined with a wavelength division multiplexing optical transmission device with reverse direction.
A SONET (Synchronous Optical NETwork)/SDH (Synchronous Digital Hierarchy) terminals, ADM devices, or prior art information communications devices 150-1, 150-2 which perform information communications, such as IP routers, are provided in the transmitting side site 141-1, and optical signals are transmitted to the wavelength division multiplexing transmission device 152 via intra-site optical fiber circuits 142-1-142-n. The intra-site optical fiber circuits 142 have a distance of the order of several mxe2x80x94several tens of kilometers, and uses for example a SONET/SDH signal format such as OC-12 (600 Mbit/s) or OC-48 (2.5 Gbit/s)
From the viewpoint of cost, IM/DD (Intensity Modulation/Direct Detection) of a laser diode operating in, for example, the 1.3 xcexcm wavelength band is used for the optical transceivers in the intra-site optical fiber circuits 142; As the signals transmitted in the intra-site optical fiber circuits have problems of wavelength band, wavelength interval, spectral purity, wavelength accuracy and dispersion tolerance, they are unsuitable for wavelength division multiplexing transmission over long distance optical fibers. Therefore, the signals are converted to plural different wavelengths (xcex1-xcexn) for inter-site wavelength division multiplexed transmission by the transmitting side transponder devices 120-1-120-n, wavelength division-multiplexed by an optical multiplexer 145, and then output to the inter-site optical fiber circuit 144-1.
The wavelength division-multiplexed optical signals transmitted to the inter-site optical fiber circuit 144-1 are relay amplified by the optical repeater 151, transmitted along the inter-site optical fiber circuit 144-2, input to a wavelength division multiplexing optical transmission device 153 on the receiving side, wavelength-demultiplexed into optical signals of wavelength xcex1-xcexn by an optical demultiplexer 146, and then respectively input to receiving side transponder devices 130-1-n.
In the prior art transmission side transponder device 120, an optical signal of wavelength xcexa is received by an intra-site transmission optical receiver 123 from the intra-site optical fiber 142, and the optical signal is then converted to an inter-site transmission signal format as a wavelength division-multiplexed optical signal (wavelength xcex1) by an inter-site transmission optical transmitter 124, and output, as shown in FIG. 6.
The wavelength of the inter-site transmission optical signal is normally in the 1.5 xcexcm band which is suitable for amplification by an optical fiber amplifier, and generally coincides with wavelength grids (50 Ghz, 100 GHz, 200 GHz interval) which are wavelengths standardized for use with WDM. Also, as the inter-site transmission distance may attain several 100-several 1000 km, external optical modulation, which is suitable for long distance transmission, is often used. For the inter-site transmission signal format, a SONET/SDH format basically identical to that used for the intra-site transmission format is widely adopted, but in recent years, a wavelength wrapper with the addition of bit error correction between transponder sections or a monitoring function has also been considered.
In the receiving side transponder device 130, an optical signal input from an inter-site side input fiber 131 is received by an inter-site transmission optical receiver 133, the optical signal is converted to an intra-site circuit signal format or the wavelength xcexa by an intra-site transmission optical transmitter 134, and output to an intra-site side output optical fiber 132, as shown in FIG. 7. These optical fibers are respectively connected to intra-site information communications devices 150-3, 150-4 via intra-site optical fiber circuits 143-1-143-n in FIG. 5.
In the prior art wavelength multiplexing transmission system, the transmission speeds on the intra-site side and inter-site side are identical in principle. For example, in the case where the transmission speed of the inter-site optical fiber circuits 142, 143 shown in FIG. 3 is 2.5 Gbyte/s, the transmission speed of an optical signal of wavelength xcexn in the inter-site optical: fiber circuit 144 was also 2.5 Gbyte/s. In this prior art device, if the information transmission amount between the information communications devices 150-2, 150-4 increases and the bit rate is increased, for example, transmission in the inter-site transmission part is difficult. As the maximum transmission distance of optical fiber transmission is inversely proportional to the square of the bit rate, when the transmission speed is as high as 10 Gbit/s or 40 Gbit/s, the maximum transmission distance of the inter-site transmission part very rapidly becomes shorter. For example, the maximum transmission distance at 2.5 Gbit/s is 600-1200 km, but as it is of the order of several tens of km at 10 Gbit/s and no more than several km at 40 Gbit/s, it is difficult to achieve long-distance transmission at these speeds. If a dispersion compensation technique or dispersion shifted fiber (DSF) is used, this value can be improved to some extent, but even then, the usual limit of the transmission distance is around 500 km at 10 Gbit/s and 40 km at 40 Gbit/s. Therefore, if the transmission speed of the intra-site optical fiber circuits 142, 143 is increased, the number of repeaters in the inter-site part must be increased, and the cost increases.
To deal with this problem, the system of FIG. 5 shows an example where the number of inter-site circuits in the information communications device 150-1 is increased to, for example, three. In this example, even if the transmission amount is increased by three times, the transmission speed in the inter-site transmission part remains the same, so the above problem can be avoided. However, the number of intra-site circuits and the number of inter-site transceivers also increases by three times, so there are other problems in that cost and intra-site circuit management difficulties increase.
Further, due to the aforesaid transmission distance problem, the intra-site transmission speed of the wavelength division multiplexing transmission device 152 is normally limited in practice to a value such as 2.5 Gbit/s-10 Gbit/s, and the speed cannot be increased without limit. Therefore, even if a faster intra-site circuit interface such as 10 Gbit/s or 40 Gbit/s were developed, it could not be applied to this wavelength division multiplexing transmission device.
It is therefore an object of this invention to provide a wavelength division multiplexing system which can transmit information at high speed without increasing the number of intra-site transmission paths which allows lengthening of the inter-site transmission path, to provide a wavelength division multiplexing optical transmission device and transponder used by this system, and to provide an information communications device using this transponder.
To achieve the above object, the wavelength division multiplexing transmission system of this invention comprises a wavelength division multiplexing transmission device connected to an information communications device by an intra-site optical fiber circuit, and a transmitting part and receiving part connected to the aforesaid wavelength division multiplexing transmission device by an inter-site optical fiber circuit. The transmitting part converts a high-speed information signal transmitted by the intra-site optical fiber circuit into plural optical signals of different wavelengths by the transponder device in the wavelength division multiplexing transmission device, and transmits an optical wavelength division multiplexed signal obtained by wavelength multiplexing the aforesaid plural optical signals of different wavelengths to the receiving part by the aforesaid inter-site optical fiber circuit. In the receiving part, the received optical wavelength division multiplexed signal is preferably wavelength-demultiplexed by a wavelength demultiplexer of the wavelength division multiplexing transmission device, the demultiplexed plural optical signals of different wavelengths are multiplexed into one high-speed optical signal by the transponder device, and transmitted to the information communications device of the receiving part by an inter-site optical fiber circuit.
The aforesaid optical transponder device of the transmitting part.comprises an intra-site transmission optical receiver which receives an information signal from the aforesaid intra-site optical fiber circuit, a demultiplexer which time divides the information signal received by the intra-site transmission optical receiver, and plural inter-site transmission optical transmitters which convert the demultiplexed signals respectively into optical signals of different wavelengths.
The aforesaid optical transponder device on the receiving side comprises plural optical receivers which receive the plural optical signals (information signals) of different wavelengths, a multiplexer which multiplexes the aforesaid plural information signals from the optical receiver into one high-speed information signal by time division multiplexing, and an intra-site transmission optical transmitter which connects the multiplexed information signal to an optical fiber circuit.
The aforesaid wavelength division multiplexing transmission device of the transmitter part comprises the transmitting side optical transponder device, and an optical multiplexer which performs optical multiplexing on the optical signals of different wavelengths demultiplexed by the aforesaid transmitting side optical transponder device.
The aforesaid wavelength division multiplexing transmission device of the receiver part comprises an optical demultiplexer which wavelength-demultiplexes the optical wavelength division multiplexed signal input from the inter-site optical fiber circuit, and the aforesaid receiving side optical transponder device which inputs the plural optical signals of different wavelengths demultiplexed by the optical demultiplexer.
In another preferred embodiment of the invention, the wavelength division multiplexing optical transmission device is provided with various functions, for example an optical repeater function, an add-drop function and a switching (routing) function, by combining the aforesaid optical transponder devices or wavelength division multiplexing optical transmission devices in the transmitting and receiving parts. The aforesaid transponder device may also be an intra-site interface device wherein the wavelength division multiplexing transmission device and information communications device are combined.
According to this invention, in the aforesaid wavelength division multiplexing optical transmission system, information signals are processed at high speed by a small number of circuits in the intra-site circuit, and the inter-site transmission path is lengthened and information is transmitted at high speed in the inter-site optical fiber circuit.
Further, the optical transponder device, wavelength division multiplexing optical transmission device and interface device of this invention may be provided, either individually or in combination, with various functions of a wavelength division multiplexing optical transmission system. For example, they may be devices having SONET/SDH/ATM multiplexing, demultiplexing, replacing, switching, fault recovery, bit rate conversion or add-drop functions, or IP packet switching, routing, broadcast or interface conversion functions.
This invention may easily be realized by means of a simple hardware construction if the multiplexing/demultiplexing in the intra-site optical circuit uses time division multiplexing/demultiplexing of information signals having one to plural bits as units, or time division multiplexing/demultiplexing with the addition of a parity signal. A device or system according to this invention having compatibility with prior art SONET/SDH devices may be realized by performing multiplexing/demultiplexing according to the SONET/SDH scheme. Maintenance and reliability of intra-site optical circuits is improved, and practical feasibility is enhanced, by adding an intra-site optical circuit monitoring information signal during multiplexing, and extracting this monitoring information signal during demultiplexing. Device and system reliability are further enhanced by adding an error correction signal before multiplexing, and extracting the error correction signal during demultiplexing.
A routing mechanism or path setting mechanism may be added by incorporating an optical signal or electrical signal switch in the device or system of this invention and changing over the signal path, whereby the application field of the invention is widened. The reliability of the invention may also be greatly improved by providing plural transceivers, reserving some of these as standbys, and switching over to standby transmission if a fault occurs. In particular, when packet information is transmitted, the aforesaid two functions may be simultaneously implemented by providing a packet information routing circuit.
Further, by giving the optical transmitter, the laser light source in the optical transmitter and the optical receiver of this invention modular constructions so that they can be individually replaced if necessary, maintenance is greatly improved even in devices having a large number of components, and the apparatus car be easily constructed. Likewise, even if a wavelength variable laser is used for the light source or the light source is supplied from outside, standby transponder devices and the types of parts used for maintenance and production can be greatly reduced. Finally, if a compensation circuit for differential group delay is provided which compensates for propagation delay lime differences between wavelength division multiplexed, transmitted optical signals, the intra-site optical circuit bandwidth can be used effectively, so the application field of the invention is widened.
These and other, objects, constructions and features of the invention will become apparent from the following embodiments thereof.